STRUCTURE 297 



ike a cuttle-fish (D) by means of its tentacles, the body 

 eing kept nearly vertical. 



It is also possible to watch a Hydra feed. It is a 

 ery voracious creature, and to see it catch and devour 

 ts prey is a curious and interesting sight. In the water 

 n which it lives are always to be found numbers of 



water-fleas," minute animals of about a millimetre or 

 ess in length, belonging to the class Crustacea (see 

 p. 386). 



Water-fleas swim very rapidly, and occasionally one 

 may be seen to come in contact with a Hydra's tentacle, 

 nstantly its hitherto active movements stop dead, and 

 t remains adhering in an apparently mysterious manner 

 o the tentacle. If the Hydra is not hungry it usually 

 iberates its prey after a time, and the water-flea may 

 hen be seen to drop through the water like a stone for 

 a short distance, but finally to expand its limbs and 

 wim off. If, however, the Hydra has not eaten 

 ecently, it gradually contracts the tentacles until the 

 >rey is brought near the mouth, the other tentacles 

 >eing also used to aid in the process. The water-flea is 

 hus forced against the* apex of the hypostome, the 

 mouth expands widely and seizes it, and it is finally 

 passed down into the digestive cavity. Hydrae can often 

 >e seen with their bodies bulged out in one or more 

 >laces by recently swallowed water-fleas. 



The precise structure of Hydra is best made out 

 >y cutting it into a series of extremely thin sections 

 ind examining them under a high power of the micro- 

 ,cope. The appearance presented by a vertical section 

 hrough the long axis of the body is shown in Fig. 76, A. 



The whole animal is seen to be built up of cells, each 

 consisting of protoplasm with a large nucleus (B-D, 



), and with or without vacuoles. As in the case of 

 nost animal cells, there is no cell- wall. 



